In preparation of a silver halide emulsion, a dopant (i.e., substances other than silver and halogen ion) is sometimes introduced into silver halide crystals, which has been well known as a doping technique. The doping technique of a transition metal has particularly been studied to improve a silver halide emulsion. For example, a transition metal compound of VIII group having a cyano has sometimes been added to a silver halide emulsion in formation of silver halide grains to improve the sensitivity of the emulsion.
Japanese Patent Publication No. 48(1973)-35373 discloses a process of forming silver halide grains in the presence of a water-soluble iron compound, which is used in an amount of 10.sup.-7 to 10.sup.-3 mol based on 1 mol of silver. The publication describes that an emulsion of hard gradation can be prepared according to the process without degrading the sensitivity.
Japanese Patent Publication No. 49(1974)-14265 discloses an emulsion containing silver halide grains having a particle size of not larger than 0.9 .mu.m. A metal compound of group VIII in an amount of 10.sup.-6 to 10.sup.-3 mol based on 1 mol of silver is added to the emulsion in formation of the grains, and the emulsion was spectrally sensitized with a merocyanine dye.
According to the process of the above-mentioned publications, an emulsion of high sensitivity can be obtained. However, a relative increase of the surface sensitivity is small, since the internal sensitivity as well as the surface sensitivity is increased in the obtained silver halide grains.
Japanese Patent Provisional Publication No. 1(1989)-21844 discloses a high sensitive silver halide emulsion containing silver halide grains that have at least two parts. The halogen compositions of the two parts are different from each other. The part that has the smallest band gap energy contains divalent iron ion. The effect of this technique is limited to the emulsion containing divalent iron ion. The publication is silent with respect to the ligand of the ion.
A transition metal compound can be added to the silver halide emulsion at the stage of grain formation. The compound may also be added to the emulsion after precipitation of silver halide grains. However, there is a considerable difference in a photographic effect between the former addition and the latter addition. In the former addition, the transition metal of the compound is introduced into the silver halide crystal as a dopant. Therefore, the transition metal can effectively change the photographic properties of the emulsion, even if a small amount of the compound is used. On the other hand, the transition metal is adsorbed on the surface of the silver halide grains in the latter addition. In this case, a relatively large amount of the transition metal compound is required to change the photographic properties of the emulsion to the same extent as the former addition. The function of the transition metal to the silver halide grains is inhibited by a protective colloid at the former addition. Accordingly, it is difficult to obtain a satisfactory photographic effect, if the transition metal is added to the emulsion at the stage of chemical sensitization. Therefore, the transition metal has been usually added as a dopant to the emulsion at the stage of silver halide grain formation. As is described above, metal doping (the former addition) is different from metal sensitization (the latter addition).
The chapter IA of Research Disclosure No. 17,643 discloses transition metal compounds, which may be added to the emulsion at the stage of precipitation of silver halide grains. On the other hand, the chapter IIIA discloses transition metal compounds, which may be added to the emulsion added during chemical sensitization.
U.S. Pat. No. 4,126,472 discloses use of iridium as a dopant attached to the surface of silver halide grain or as a surface modifier for silver halide. According to the description of the patent, silver halide emulsion is sensitized in the presence of a water-soluble iridium salt. The amount of the salt is 10.sup.-6 to 10.sup.-4 mol based on 1 mol of silver halide. However, U.S. Pat. No. 4,126,472 is silent with respect to hexa-coordinated cyano-complex.
European Patent No. 0,242,190 describes that silver halide emulsion containing grains formed in the presence of a complex of trivalent rhodium having three, four, five or six cyano ligands. In the emulsion described in the publication, low intensity reciprocity law failure is reduced.
U.S. Pat. No. 3,690,888 discloses a process for preparing silver halide containing multivalent metal ions. In the process, silver halide is formed in the presence of protective colloid mainly comprising acrylic polymer. U.S. Pat. No. 3,690,888 further describes that the multivalent metal ions include bismuth, iridium, lead and osmium. However, U.S. Pat. No. 3,690,888 is silent with respect to hexa-coordinated cyano complex.
The above-mentioned publications do not disclose that the ligands are introduced into a grain together with the transition metal. Further, they are silent with respect to regulation of the ligand and the effect of the transition metal complex.
European Patents No. 0,336,190 and No. 0,336,426 and Japanese Patent Provisional Publications No. 2(1990)-20853 and No. 2(1990)-20854 disclose silver halide emulsions having excellent characteristics. The emulsions can be obtained by using hexa-coordinated complex of rhenium, ruthenium, osmium or iridium having at least four cyano ligands. In the emulsions described in these publications, low intensity reciprocity law failure is reduced, and the sensitivity and gradation of the emulsion are stable.
Each of European Patent No. 0,336,427 and Japanese Patent Provisional Publication No. 2(1990)-20852 discloses a silver halide emulsion of a controlled sensitivity. In the emulsion, low intensity reciprocity law failure is reduced without decreasing the sensitivity of a middle intensity by using a hexa-coordinated complex of vanadium, chromium, manganese, iron, ruthenium, osmium, rhenium or iridium having nitrosyl or thionitrosyl ligand.
Each of European Patent No. 0,336,689 and Japanese Patent Provisional Publication No. 2(1990)-20855 also discloses a silver halide emulsion of a controlled sensitivity. In the emulsion, low intensity reciprocity law failure is reduced by hexa-coordinated rhenium complex. The complex has six ligands selected from halogen, nitrosyl, thionitrosyl, cyano water and thiocyan.
Japanese Patent Provisional Publication No. 3(1991)-118535 discloses a hexa-coordinated transition metal complex having carbonyl group as one of the ligands. Further, Japanese Patent Provisional Publication No. 3(1991)-118536 discloses an emulsion containing a hexa-coordinated transition metal complex in which two of the ligands are oxygen atoms.
U.S. Pat. No. 5,132,203 discloses high sensitive tabular grains, which contains a hexa-coordinated complex of a metal of VIII group having at least four cyano ligands in the subsurface. The surface of the grain (20 to 350 .ANG.) does not contain the complex. European Patent No. 0,508,910 discloses a silver halide emulsion, wherein the subsurface of the silver halide grain is doped with a hexa-coordinated iron complex. The surface of the grain (20 to 350 .ANG.) is not doped with the iron complex. These patents suggest that the hexa-coordinated cyano complex is doped near the surface of the grain to obtain a high sensitivity, but the complex is not preferably present in the surface. Accordingly, they teach that the subsurface of the grain is doped with the hexa-coordinated metal cyano-complex and they are silent with respect to the surface doping of the complex.